An ion source for an accelerator or the like generally comprises a vacuum chamber in which a low pressure gas, such as hydrogen, deuterium or the like, is ionized to produce an ionized gas plasma. It is often desirable to produce a plasma having a high ion density so that the accelerator may be supplied with a beam having a high density of charged or neutral particles, such as gas ions, atoms or molecules.
One common method of producing a high density plasma in an ion source is to provide thermionic cathode filaments which emit a copious supply of electrons, which then may be accelerated to produce an ionized gas plasma. This approach has the disadvantage that thermionic cathode filaments often have a very short operating life of only a few hours, for example. Moreover, the electrically heated filaments produce considerable heat which may cause operating problems.
Another method of producing a dense ionized gas plasma is to supply radio frequency power to the vacuum space. Generally, a small thermionic cathode filament is provided to emit electrons so that there is initial ionization of the ionizable gas, which then derives additional energy from the radio frequency power so that a dense ionized gas plasma is produced. Thus, in a manner of speaking, the radio frequency power heats or increases the energy level of the ionized gas so that a dense plasma is produced.
The radio frequency power is supplied to the ion source by an RF antenna in the vacuum chamber. The RF antenna coil has power lead-ins which extend through seals in the walls of the vacuum chamber and are connected to an RF amplifier or other generator, outside the vacuum chamber. The RF power often has a frequency in the range of one to two megahertz.
The RF antenna often takes the form of an elongated electrical conductor formed into a coil.
Frequently, the antenna coil may be made of copper tubing.
Problems have been encountered with such radio frequency ion source antenna coils. When the antenna coil is made of bare metal, such as copper, sparking or arcing may occur in the vacuum chamber, both between the turns of the coil, and also between the coil and various electrodes which may be employed in the ion source. When the antenna coil is operated at high power levels, the RF voltage between different portions of the coil may be quite high. Moreover, electrodes may be employed in the ion source to produce accelerating voltages which are quite high, so that sparking or arcing may occur.
When a bare antenna coil is employed in an ion source, problems are often encountered with sputtering of the copper or other metal from the antenna coil, due to ion bombardment of the antenna coil. The sputtered copper or other metal is deposited on other surfaces within the vacuum chamber of the ion source, and may cause problems, such as current leakage or short circuits between electrodes.
An attempt has been made to deal with these problems of voltage breakdown, sparking, arcing and sputtering by covering the bare antenna coil with sleeving material made of woven glass or quartz fibers, to act as electrical insulation. This approach reduces sputtering but does not eliminate sputtering as a problem. Moreover, the woven glass or quartz sleeving provides only limited protection against voltage breakdown, sparking and arcing, without eliminating them as problems.
Moreover, the woven glass or quartz sheathing introduces the additional problem of causing the evolution of contaminating gases, such as oxygen and water vapor, which are driven out of the woven glass or quartz material during the operation of the ion source, largely due to the heat generated in the ion source during normal operation.
The principal object of the present invention is to provide a method of making a radio frequency ion source antenna which deals much more effectively with the problems of voltage breakdown, sparking, arcing and the evolution of contaminating gases.